Improve CLI recording rendering - #14507

Follow-up to #14475.

Description

When trying out the skill in #14475, I found that the font chosen before execution could not render the braille loader. Font selection is not used to execute or capture the command, so this PR keeps fonts out of execution contracts and preflight receipts. The evidence skill must select the primary font explicitly with --font when it renders, and repeated --font-fallback options define the complete ordered fallback chain without modifying or rerunning the capture. Each completed rendering records the exact primary, auto-loaded style companion, and fallback font paths and hashes that could produce its pixels.

I also was confused when reviewing the recording because QuickTime's on-hover header was obscuring commands, and I thought the rendering pipeline was broken. This PR introduces a consistent framed 48px safe area.

How did you test this change?

Using the immutable capture from an organization-owned-fork scenario, I rendered a new 1416x1008 session video with Menlo and Apple Braille selected at render time. The video shows the Braille spinner, expected GraphQL failure, validation, cleanup, overview, and chapter boundaries, with terminal and overview content inside a 48px safe area. This demonstrates that missing glyph coverage and presentation styling can be corrected without executing the recorded GitHub operations again.

Key points

• Execution contracts and preflight receipts contain no font selection; contract font fields are rejected.
• Every rendering requires an explicit --font; repeated --font-fallback values define the complete ordered presentation-only fallback chain.
• Each rendering records the primary font in rendering.font, auto-loaded style companions in rendering.fontStyles, and glyph fallbacks in rendering.fontFallbacks, including path, family, and SHA-256.
• Unavailable render-time fonts fail rendering without changing the recorded case outcome or captured evidence.
• The safe-area frame increases final media dimensions by 96 pixels in each axis rather than cropping or scaling terminal content.

Notes for reviewers

Start with .github/skills/cli-exercise/tool/internal/engine/contract.go for the execution-contract boundary and evidence/main.go for mandatory explicit render-time font selection. fontutil/font.go and evidence/render.go contain complete primary/style/fallback provenance. raster.go and overview.go contain the framed presentation, while fonts_test.go covers capture preservation and rendering metadata. This PR is intentionally layered on #14475.

Authorship and follow-up

Who wrote this:

• A human wrote it.
• An agent wrote it under close human direction.
• An agent wrote it independently, and no human has guided the implementation beyond the initial prompt.

Who answers review comments:

• @williammartin will read and reply directly. Name the account.
• An agent will draft replies and @username will read them before they are posted.
• Nobody has explicitly committed to replying.